NEW DELHI: At least 20 people were killed and five others feared trapped following a blast caused by a methane gas leak inside an under-construction tunnel in northeastern India, police said Wednesday.

The incident took place on Monday. According to officials, 25 people were inside the tunnel at the time of the blast.

“Out of those 25 trapped inside the tunnel, 20 are confirmed dead,” a senior police official in Namchi Sonam D Bhutia told media.

Reports pouring in from the site said the blast took place inside a 7 km under-construction tunnel being operated by India’s state-run National Hydroelectric Power Corporation (NHPC). The victims include high-level personnel from the NHPC, who were on a routine inspection inside the tunnel.

NHPC said in a statement that the incident was caused by the “sudden explosion of trapped/embedded suspected methane gas within the rocks, resulting in dense smoke and toxic gases.”

The hydropower company said a detailed investigation will be conducted to ascertain the cause of the incident.